Each of Patent Literatures 1 and 2 describes a focused ion beam (FIB) device in which a gas field ion source (GFIS) is mounted and which uses ions of gas, such as hydrogen (H2), helium (He), and neon (Ne). The gas focused ion beam (gas FIB) device has the advantage of preventing that a sample is contaminated with gallium (Ga: metal) as in the case of a Ga focused ion beam (Ga-FIB) emitted from a liquid metal ion source (LMIS) which is often used at present. In addition, in the GFIS, the energy spread of gas ions extracted from the GFIS is narrow, and the source size is small. Therefore, in the GFIS, a finer beam can be formed as compared with the case of the Ga-FIB.
On the other hand, Patent Literature 3 and Non Patent Literature 1 disclose that an improvement in ion source characteristics, such as an increase in the angular current density, can be achieved in such a manner that a minute protruding portion is formed at the apex of the emitter tip of the GFIS, or that atoms arranged at the apex of the emitter tip are reduced to several atoms or less. As an example in which such minute protrusion (hereinafter referred to as “nanotip”) is formed at the apex of the emitter tip, Non Patent Literature 1 and Patent Literature 2 disclose that a minute protrusion is formed from an emitter tip made of tungsten (W: metal) by using field evaporation. In this case, a nanotip is one or three atoms terminated along the [111] direction of the W single crystal. Non Patent Literature 2 and Patent Literature 4 disclose that a nanotip is formed by using a second metal (for example, a noble metal, such as iridium (Ir) or platinum (Pt)) different from the first metal material (for example, W) of the emitter tip. In this case, the nanotip is a pyramid having the surface of the second metal formed at the [111] direction terminal portion of the W single crystal.
Patent Literature 5 describes a method in which the angular current density is increased by increasing the curvature of the apex of the emitter tip in the GFIS without using the nanotip.